Direct observation of d-orbital holes and Cu–Cu bonding in Cu2O

A striking feature of metal oxide chemistry is the unusual electronic and chemical behaviour of Cu(I) and Ag(I): a case in point is that detailed understanding of Cu–O bonding is essential to the theory of high-temperature copper oxide superconductors. Both cations are usually coordinated in a linear fashion to two oxygens, particularly for Cu(I). In many compounds, the Cu(I) and Ag(I) cations also adopt close-packed (and related) configurations with short metal–metal distances that are strongly suggestive of the occurrence of metal–metal bonding1,2 despite their formal nd10 configuration. Such observations have been explained3,4 by invoking the participation in bonding of electronic orbitals of higher principal quantum number—that is, (n + 1)s and (n + 1)p—accompanied by the creation of d-orbital holes on the metal ion. To test this hypothesis, we have used a recently developed method of quantitative convergent-beam electron diffraction5 combined with X-ray diffraction to map the charge-density distribution in the simple oxide Cu2O, the results of which we then compare with electronic-structure calculations. We are able to image directly the d holes on the copper atoms, and also demonstrate the existence of Cu–Cu bonding in this compound.